Strange values from ADC

[tchitchi]

Hello,

I am using simple_adc__pca10028 from 8.0.0 SDK to read values from my sensor, its a kind of tensometer. My configurations are:

• NRF_ADC_CONFIG_RES_10BIT, --- 10 bits of resolution

• NRF_ADC_CONFIG_SCALING_INPUT_ONE_THIRD,--- 1/3 of supply value

• NRF_ADC_CONFIG_REF_SUPPLY_ONE_HALF --- Am I thinking right, is it 3V/2 = 1,5V?

I suppose my reference voltage should be now 1,5V*3 = 4,5V, right?

I am sending readings to PC through UART, and my Termite shows values ranging from 175 to 680. I can understand that 680 = (3V/4.5V)*1023, but why 175? Why the lowest values aren't equal to zero? I mean lowest, when the current isn't flowing through my sensor.

Reading the GND values gives me 0, but unplugged wire still 175. please help

best regard, tchitchi

[RK]

"Unplugged wire"? What does that mean? Just a floating input with no reference? A floating wire is going to have any random kind of voltage on it, why would you expect zero? What voltage do you measure on your connected sensor with a voltmeter when it's at the low point? Are you using a common ground for the nRF chip and the sensor?